The exhaust gas emitted from an internal combustion engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions, typically disposed on catalyst supports or substrates disposed within the exhaust system of an internal combustion engine are provided to convert certain or all of these exhaust gas constituents into non-regulated exhaust gas components. For example, exhaust systems for internal combustion engines may include one or more of a precious metal containing oxidation catalyst (“OC”) for the reduction of CO and engine-out HC, a selective catalyst reduction catalyst (“SCR”) for the reduction of NOx and a particulate filter device (“PF”) for the removal of particulate matter.
An exhaust gas treatment technology in use for high levels of particulate matter reduction, the PF may utilize one of several known exhaust gas filter structures that have displayed effectiveness in removing the particulate matter from the exhaust gas. Such exhaust gas filter structures may include, but are not limited to ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications.
The exhaust gas filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates in the exhaust gas filter will have the effect of increasing backpressure in the exhaust system that is experienced by the internal combustion engine. To address backpressure increases caused by the accumulation of exhaust gas particulates in the exhaust gas filter, the PF is periodically cleaned, or regenerated. Regeneration of a PF in vehicle applications is typically automatic and is controlled by an engine or other suitable controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the PF filter, typically by heating the engine exhaust gas, to levels that are often at or above 600° C. in order to burn the accumulates particulates.
One method of generating the exhaust gas temperatures required in the exhaust system for regeneration of the PF is to deliver unburned HC to an oxidation catalyst device disposed upstream of the PF. The HC may be delivered to the exhaust system by direct fuel injection into the exhaust system or may be achieved by “over-fueling” the internal combustion engine resulting in unburned HC's exiting the engine in the exhaust gas. The HC is oxidized in the oxidation catalyst device in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas travels downstream to the PF and burns the particulate accumulation.
A disadvantage to this method of regeneration is that it is typically desirable to locate the NOx reduction device within the exhaust system at a location that is upstream of the PF in order to protect the device from high temperature excursions caused by PF regenerations. Similarly it is desirable to locate the NOx reduction device downstream of the OC to protect the sensitive SCR catalysts from poisoning by HC adsorption. If the OC is used to heat the exhaust gas, as described, during regeneration of the PF, NOx reduction across the NOx reduction device may be substantially reduced due to high temperatures generated by the oxidation of the HC in the OC. Additionally, thermal degradation of the NOx reduction device may be more pronounced resulting in rapid aging of the catalyst and lower than desired durability.